Copy Ultraedit's "file dissapeared" semantics

Description

"Would you like to CLEAR window" is less stupid than the previous behaviour, but still not good enough.

If a file on disk dissapears, you should be asked if you want to keep the file open. If so, then the file should become a "new" unsaved file with no direct ties to the filesystem, similar to the existing "Unsaved 1" type files.